Swiss chard (Beta vulgaris) is a leafy vegetable belonging to the family Amaranthaceae. All cultivated Beta vulgaris varieties fall into the subspecies Beta vulgaris subsp. vulgaris. The wild ancestor of these cultivated varieties is Beta vulgaris subsp. maritima, commonly known as the sea beet, and is found throughout the Mediterranean, the Atlantic coast of Europe, the Near East, and India. Beta vulgaris has four different cultivated forms: the common garden beet, also called beetroot or table beet, is a vegetable grown for its roots; the sugar beet is used to produce sugar; mangelwurzel is a fodder crop; and Swiss chard, also known as chard, leaf beet or silverbeet, is grown for its nutrient-rich leaves. Though nowadays the generally accepted name for Swiss chard is Beta vulgaris subsp. vulgaris, its taxanomic rank has changed many times, and Swiss chard is also occasionally indicated as Beta vulgaris var. vulgaris or Beta vulgaris var. cicla. 
Swiss chard leaves are an excellent source of vitamins such as A, C, and K, and a good source of minerals such as iron, magnesium and potassium. Fresh young “baby leaf” Swiss chard can be used raw in salads, while mature Swiss chard leaves and stalks are typically cooked or sautéed. Swiss chard is not only a healthy but also an attractive leafy vegetable, with a lot of variation in coloring. Depending on the variety the leaf blade may be green, dark green or reddish in color, while the petiole (also called the leaf stalk) and major veins (midrib and secondary veins) of the leaf may be white, green, yellow, orange, red or even purple. The yellow to reddish-purple colors are caused by the presence of different betalain pigments. There are two classes of betalains: (a) betacyanins that include the reddish to violet betalain pigments and (b) betaxanthins that include the yellow to orange betalain pigments.
Swiss chard leaves may show weak to strong blistering of the leaf blade. Strong blistering of the leaf blade is preferred in the fresh market because those leaves pack looser than leaf varieties with weak or medium blistering of the leaf blade and are less likely to wilt or turn yellow.
Downy mildew is a harmful disease that may cause serious losses to Swiss chard crops. Since young leaves are particularly affected, this can be a major issue in Swiss chard “baby leaf” production. The causal agent of downy mildew on various Amaranthaceae, including Swiss chard, is regarded as a single species, Peronospora farinosa. In particular, Peronospora farinosa f. sp. betae (formerly called Peronospora schactii) infects Swiss chard. Initial symptoms of downy mildew consist of light green leaf spots on the upper leaf surfaces. Under moist conditions an initially whitish and then dull to violet-gray hyphal growth appears on the lower surface and sometimes upper surface of the leaves and sporangia become visible. Affected leaves may wilt and die. Since cotyledons and young leaves are particularly affected, this can be a major issue in Swiss chard “baby leaf” production. In older Swiss chard plants the growing point may become infected and may cause the leaves to become thickened, distorted, light green and often twisted with downward-curled margins. Flower parts may also be infected, causing mycelium and oospores to develop within seed clusters. Under favorable conditions the infection can spread very rapidly, resulting in widespread crop damage. The optimal temperature for formation and germination of P. farinosa f. sp. betae spores is around 12° C., and it is facilitated by a high relative humidity greater than 85%. When spores are deposited on a humid leaf surface they can readily germinate and infect the leaf. The optimum temperature range for infection is 7-15° C. The disease incubation period ranges from 4 to 32 days, depending on environmental conditions. Peronospora farinosa f. sp. betae survives as oospores and mycelium in crop residues, in wild and volunteer Beta species, and in seeds. Although some fungicide treatments may be effective, they are costly and cause ecological pollution.
Cercospora leaf spot, caused by the pathogen Cercospora beticola is one of the most important diseases that infects Swiss chard, and can also grow on many weed species, spinach and most wild Beta species. Initial symptoms are small, angular leaf spots that are brown or tan with purple to red borders. Lesions at maturity appear gray-colored to dark tan with a brown to purple border. In severe cases the spots can coalesce and cause large sections or entire leaves to die. Cercopora leaf spot may be recognized by the presence of tiny black dots (pseudostromata) that form in lead substomatal cavities with grayish-tan lesions. These pseudostromata produce conidiophores that bear the spores of Cercospora beticola. The pathogen survives mainly in plant debris, but can also survive in seeds. When moisture is sufficient, new spores are formed and spread via rain-splash and wind to new leaves or plants. Cercospora leaf spot can result in significant losses, especially in late summer when conditions are favorable (high temperatures, high humidity, long leaf wetness periods at night) as infected Swiss chard leaves become unmarketable. Although fungicide treatment may be effective, this must be considered carefully as C. beticola populations have been known to develop a resistance to major classes of fungicides.
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